In the wireless communications industry, market demands for ubiquitous network access to information and services has been met with rapid and widespread development of wireless network applications for wireless PAN (personal area network), wireless LAN (local area network), wireless WAN (wide area network), cellular networks, and other types of wireless communication systems. Moreover, recent advances in semiconductor technologies and microwave and millimeter-wave planar antenna circuit technologies have made it possible to combine the solid-state devices with the planar antennas to produce more compact, reliable and wide-band microwave and millimeter-wave devices.
In general, micros trip antenna technologies are commonly used for integrated RF solutions as microstrip antennas (or printed antennas) such as microstrip patch antennas (or patch antenna) allow for low profile, low cost and reliable antenna designs. As is well known in the art, a patch antenna is typically fabricated by patterning an antenna radiator element on one side of an insulating dielectric substrate that has a continuous metal layer (ground plane) on the opposite side of the substrate. A single patch antenna provides a maximum directive gain of around 6-9 dBi. For applications such as wireless USB, where the operating distance is limited to about a meter, a single patch antenna with about 7 dBi of gain at an operating frequency of 60 GHz will provide the necessary antenna gains. However, for distances of 10 meters or more or other point-to-point applications, antenna gains as high as 40 dBi are needed depending on the application. To achieve such high gain, antenna arrays with as many as 256 radiating elements are typically required.
Planar antenna arrays are generally designed with 2D array of radiating elements, such as microstrip patches, dipoles, folded dipoles, slots, etc., that are interconnected using a microstrip feed line network such as a corporate feed network, a series feed network or a combination of corporate and series feed networks. By way of example, FIG. 1 illustrates a conventional architecture of a microstrip patch antenna array (100) having a 2D array (4×4) of 16 microstrip radiator patch elements (110) are uniformly arranged in two rows of four patch elements (110). It is assumed that the array (100) is a formed on one side of a dielectric substrate having a ground plane disposed on an opposite side thereof. The array of patch elements (110) are interconnected using a corporate structured feed line network comprising successive divisions of plurality of microstrip lines (120˜123) to connect each patch element (110) to a common feed point node (FP) in parallel. The corporate feed structure comprises a central main microstrip line (120) and a plurality of interconnecting branch lines (121, 122 and 123) that connect the main line (120) to each of the patch elements (110). The branch lines (121) are connected to the ends of the main line (120) and extend at right angles to the main line (120). The branch lines (122) are connected to the ends of the branch lines (121) and extend at right angles to the ends of the branch lines (121). The branch lines (123) are connected to the ends of the branch lines (122) and each branch line (123) feeds a pair of patches (110). Each patch element (110) is feed by the branch line (123) in the array on the same side edge at a center point of the side.
In the conventional corporate-structured microstrip feed line structure of FIG. 1, the RF signal input/output point (FP) is located at the center point along the main feed line (120) in the central region of the array (100). As shown in FIG. 1, the feed point (FP) is shown located at the origin of orthogonal dotted lines X-X and Y-Y. The array (100) is arranged to be a mirror image about line Y-Y, and a substantially symmetrical about line X-X, for purposes of providing an equal transmission line path length from the common feed point FP to each of the individual radiator patches (110) along portions of the lines (120-121-122-123). With this arrangement, all patches (110) are fed in a common phase and the power of the RF signal is split at each juncture between branches. Conventional impedance matching techniques are utilized at each juncture and at each connection point with an individual radiator patch (110) to minimize reflection. Moreover, the spacing Dx between patch elements (110) in the row direction are typically selected so that the elements are electrically separated by a one-half of the free-space wavelength or some value less than one free-space wavelength. Similarly, the spacing Dy between the feed edges of the patch elements (110) in the column direction is typically selected to be one-half of the free-space wavelength. This arrangement ensures that current delivered to each patch (110) from, the feed point FP has the same amplitude and phase to thereby enhance the distribution/combination of microwave power to/from the patch element (110) at the feed point (FP) and to avoid grating lobes.
Although FIG. 1 illustrates an array of 16 patches, in certain wireless communication applications, a large number of radiating patch elements (e.g., 256 patches) are needed to achieve high gain, resulting in large arrays. As the size of the antenna array increases (and size of microstrip network) or as the operating frequency increases, the use of microstrip feed line networks are problematic. However, as the number of patches used in a microstrip antenna array increases, it may be difficult to obtain the desired gain characteristics dues to large feeding losses that result from the antenna feed network. Indeed, a corporate microstrip feed network, like any feed network, will result in a certain amount of loss of antenna efficiency and gain, due to losses in the microstrip lines, undesired radiation from, the lines, and mutual coupling between patches via surface waves. In this regards, the architecture of the feed network is a critical factor in the design process, especially for large arrays.
Moreover, it is well known that a microstrip antenna structure having a microstrip feed network and patch array on the same level (on the same substrate surface) cannot be optimized simultaneously because the requirements for microstrip antennas and microstrip transmission lines are different. For microstrip patch antennas, the relative permittivity and thickness of the substrate largely determines the electrical characteristics of the antenna a low dielectric constant substrate enhances the radiation efficiency of the antenna while a thicker substrate increases antenna bandwidth. However, lower dielectric constant substrates and thicker substrates can result in spurious radiation from microstrip line and step discontinuities. For applications at operating frequencies lower than millimeter wave frequencies (e.g., less than about 30 GHz), microstrip feed lines can be readily implemented on printed circuit board (PCB) with substrate as thin as 300 microns. For low frequency application, microstrip lines are well behaved and have acceptable electrical characteristics, e.g., acceptable dispersions, relatively constant characteristic impedance over operating band of interest, and low coupling between feed lines and radiating elements (due to weak, fringe fields).
However, for applications at millimeter wave operating frequencies and beyond, microstrip patch antenna arrays cannot be easily implemented on a substrate as thin as 100 um or thinner due to mechanical reliability and limitations of the manufacturing process, as is understood by those of ordinary skill in the art. As a result the substrate is usually too thick for microstrip lines operating at mmWave and higher frequencies. Due to the too thick substrate thickness, microstrip line impedance changes significantly with frequencies due to dispersion. In addition, the microstrip line fringe field is very strong because of the lower ratio of the microstrip line width to the substrate thickness. Under these conditions, the microstrip line does not properly operate as a feed line, but operates as a radiating element as well. Therefore, for millimeter wave frequencies and beyond, it is virtually impossible to design efficient planar antenna arrays with coplanar microstrip lines, even for small arrays with, e.g., four elements.